FN-8683: remove unreachable SQLite task polling replica

Document PostgreSQL task-deletion observation and remove the obsolete SQLite polling path.

- Remove polling state, replica emissions, and activity-log suppression from TaskStore.
- Retain backend-aware cache warming while documenting the transactional-outbox follow-up.
- Add tombstone and soft-delete abort coverage across core and engine lanes.

Files changed:
 docs/architecture.md                               |   3 +-
 ...tgres-cross-process-task-deleted-observation.md | 128 ++++++++++++++++
 docs/storage.md                                    |   3 +-
 .../task-delete-nonblocking-cleanup.test.ts        |  54 +++++++
 .../task-deleted-polling-replica-tombstone.test.ts |  57 +++++++
 .../task-updated-lanes-emit-surfaces.test.ts       |  26 ----
 packages/core/src/store.ts                         |  13 +-
 packages/core/src/task-store/lifecycle-ops.ts      | 168 ++-------------------
 packages/core/src/task-store/task-artifacts-ops.ts |   4 -
 .../__tests__/executor-soft-delete-abort.test.ts   |  15 ++
 .../src/__tests__/triage-soft-delete-abort.test.ts |  14 ++
 11 files changed, 284 insertions(+), 201 deletions(-)

Fusion-Task-Id: FN-8683
Fusion-Task-Lineage: a052db0c-b6fc-4b05-b6b2-f8217b56ded0
Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>

packages/engine/src/__tests__/executor-soft-delete-abort.test.ts

@@ -124,6 +124,21 @@ describe("TaskExecutor soft-delete aborts", () => {
     expect((executor as any).activeSubagentSessions.has("FN-TEST-4")).toBe(false);
   });
 
+  it("receives the unchanged TaskStore delete payload when metadata is present", async () => {
+    const { store, emit } = createEventedStore();
+    const executor = new TaskExecutor(store, "/tmp/test");
+    const abort = vi.fn().mockResolvedValue(undefined);
+
+    (executor as any).activeSessions.set("FN-TEST-META", {
+      session: { abort, dispose: vi.fn() },
+      seenSteeringIds: new Set<string>(),
+    });
+    emit("task:deleted", { ...makeTask("FN-TEST-META"), deletedAt: "2026-08-01T09:59:00.000Z" }, { githubIssueAction: "auto" });
+    await (executor as any).pendingTaskDisposals.get("FN-TEST-META");
+
+    expect(abort).toHaveBeenCalledTimes(1);
+  });
+
   it("is a silent no-op when the deleted task has no active surfaces", () => {
     const { store, emit } = createEventedStore();
     const executor = new TaskExecutor(store, "/tmp/test");

packages/engine/src/__tests__/triage-soft-delete-abort.test.ts

@@ -73,6 +73,20 @@ describe("TriageProcessor soft-delete aborts", () => {
     processor.stop();
   });
 
+  it("receives the unchanged TaskStore delete payload when metadata is present", async () => {
+    const { store, emit } = createEventedStore();
+    const processor = new TriageProcessor(store, "/tmp/root");
+    const abort = vi.fn().mockResolvedValue(undefined);
+
+    processor.start();
+    (processor as any).activeSessions.set("FN-TEST-META", { abort, dispose: vi.fn() });
+    emit("task:deleted", { id: "FN-TEST-META", deletedAt: "2026-08-01T09:59:00.000Z" }, { githubIssueAction: "auto" });
+    await Promise.resolve();
+
+    expect(abort).toHaveBeenCalledTimes(1);
+    processor.stop();
+  });
+
   it("is a no-op for unknown soft-deleted ids", () => {
     const { store, emit } = createEventedStore();
     const processor = new TriageProcessor(store, "/tmp/root");